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The reactivities of the adamantane‐like heteronuclear vanadium‐phosphorus oxygen cluster ions [VxP4?xO10].+ (x=0, 2–4) towards hydrocarbons strongly depend on the V/P ratio of the clusters. Possible mechanisms for the gas‐phase reactions of these heteronuclear cations with ethene and ethane have been elucidated by means of DFT‐based calculations; homolytic C? H bond activation constitutes the initial step, and for all systems the P? O. unit of the clusters serves as the reactive site. More complex oxidation processes, such as oxygen‐atom transfer to, or oxidative dehydrogenation of the hydrocarbons require the presence of a vanadium atom to provide the electronic prerequisites which are necessary to bring about the 2e? reduction of the cationic clusters.  相似文献   

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The bis(ethylene) IrI complex [TpIr(C2H4)2] ( 1 ; Tp=hydrotris(3,5‐dimethylpyrazolyl)borate) reacts with two equivalents of aromatic or aliphatic aldehydes in the presence of one equivalent of dimethyl acetylenedicarboxylate (DMAD) with ultimate formation of hydride iridafurans of the formula [TpIr(H){C(R1)?C(R2)C(R3)O }] (R1=R2=CO2Me; R3=alkyl, aryl; 3 ). Several intermediates have been observed in the course of the reaction. It is proposed that the key step of metallacycle formation is a C? C coupling process in the undetected IrI species [TpIr{η1O‐R3C(?O)H}(DMAD)] ( A ) to give the trigonal‐bipyramidal 16 e? IrIII intermediates [TpIr{C(CO2Me)?C(CO2Me)C(R3)(H)O }] ( C ), which have been trapped by NCMe to afford the adducts 11 (R3=Ar). If a second aldehyde acts as the trapping reagent for these species, this ligand acts as a shuttle in transfering a hydrogen atom from the γ‐ to the α‐carbon atom of the iridacycle through the formation of an alkoxide group. Methyl propiolate (MP) can be used instead of DMAD to regioselectively afford the related iridafurans. These reactions have also been studied by DFT calculations.  相似文献   

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Undue influence : N‐heterocyclic carbenes (NHCs) were found to reduce the strength of the B? H bonds of borane by a surprisingly large amount upon the formation of NHC–BH3 complexes. This property was exploited in the development of a suite of NHC–borane complexes for the reduction of xanthates in radical‐mediated Barton–McCombie‐type deoxygenation reactions (see scheme). AIBN=azobisisobutyronitrile, Bn=benzyl.

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High‐valent manganese(IV or V)–oxo porphyrins are considered as reactive intermediates in the oxidation of organic substrates by manganese porphyrin catalysts. We have generated MnV– and MnIV–oxo porphyrins in basic aqueous solution and investigated their reactivities in C? H bond activation of hydrocarbons. We now report that MnV– and MnIV–oxo porphyrins are capable of activating C? H bonds of alkylaromatics, with the reactivity order of MnV–oxo>MnIV–oxo; the reactivity of a MnV–oxo complex is 150 times greater than that of a MnIV–oxo complex in the oxidation of xanthene. The C? H bond activation of alkylaromatics by the MnV– and MnIV–oxo porphyrins is proposed to occur through a hydrogen‐atom abstraction, based on the observations of a good linear correlation between the reaction rates and the C? H bond dissociation energy (BDE) of substrates and high kinetic isotope effect (KIE) values in the oxidation of xanthene and dihydroanthracene (DHA). We have demonstrated that the disproportionation of MnIV–oxo porphyrins to MnV–oxo and MnIII porphyrins is not a feasible pathway in basic aqueous solution and that MnIV–oxo porphyrins are able to abstract hydrogen atoms from alkylaromatics. The C? H bond activation of alkylaromatics by MnV– and MnIV–oxo species proceeds through a one‐electron process, in which a MnIV–‐oxo porphyrin is formed as a product in the C? H bond activation by a MnV–oxo porphyrin, followed by a further reaction of the MnIV–oxo porphyrin with substrates that results in the formation of a MnIII porphyrin complex. This result is in contrast to the oxidation of sulfides by the MnV–oxo porphyrin, in which the oxidation of thioanisole by the MnV–oxo complex produces the starting MnIII porphyrin and thioanisole oxide. This result indicates that the oxidation of sulfides by the MnV–oxo species occurs by means of a two‐electron oxidation process. In contrast, a MnIV–oxo porphyrin complex is not capable of oxidizing sulfides due to a low oxidizing power in basic aqueous solution.  相似文献   

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Skillfully attached! meso–meso‐Linked diporphyrins can be efficiently and selectively functionalized with multiple unsaturated carboxylic acid groups through iridium and rhodium catalyses. This post‐modification strategy allows fine‐tuning of energy levels of each porphyrin unit.

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In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L? H)]+ (L=2,2′‐bipyridine (bipy), 2‐phenylpyridine (phpy), and 7,8‐benzoquinoline (bq)) with linear and branched alkanes CnH2n+2 (n=2–4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L? H)]+/C2H6, loss of C2H4 dominates clearly over H2 elimination; however, the mechanisms significantly differs for the reactions of the “rollover”‐cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen‐atom transfer from C2H6 to [Pt(bipy? H)]+, followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)]+, for the phpy and bq complexes [Pt(L? H)]+, the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L? H)(H2)]+ as the ionic product accounts for C2H4 liberation. In the latter process, [Pt(L? H)(H2)(C2H4)]+ (that carries H2 trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C2H4 rather than H2 is ejected. For both product‐ion types, [Pt(H)(bipy)]+ and [Pt(L? H)(H2)]+ (L=phpy, bq), H2 loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy? H)]+ with the higher alkanes CnH2n+2 (n=3, 4), H2 elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C3H5)(bipy)]+. In the reactions of [Pt(L? H)]+ (L=phpy, bq) with propane and n‐butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of “rollover”‐cyclometalated [Pt(bipy? H)]+ with CnH2n+2 (n=2–4) less than 15 % of the generated product ions are formed by C? C bond‐cleavage processes, this value is about 60 % for the reaction with neo‐pentane. The result that C? C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2‐elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1‐ and 1,3‐elimination pathways are only of minor importance.  相似文献   

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Vanadium–silver bimetallic oxide cluster ions (VxAgyOz+; x=1–4, y=1–4, z=3–11) are produced by laser ablation and reacted with ethane in a fast‐flow reactor. A reflectron time of flight (Re‐TOF) mass spectrometer is used to detect the cluster distribution before and after the reactions. Hydrogen atom abstraction (HAA) reactions are identified over VAgO3+, V2Ag2O6+, V2Ag4O7+, V3AgO8+, V3Ag3O9+, and V4Ag2O11+ ions, in which the oxygen‐centered radicals terminally bonded on V atoms are active sites for the facile HAA reactions. DFT calculations are performed to study the structures, bonding, and reactivity. The reaction mechanisms of V2Ag2O6++C2H6 are also given. The doped Ag atoms with a valence state of +1 are highly dispersed at the periphery of the VxAgyOz+ cluster ions. The reactivity can be well‐tuned gradually by controlling the number of Ag atoms. The steric protection due to the peripherally bonded Ag atoms greatly enhances the selectivity of the V–Ag bimetallic oxide clusters with respect to the corresponding pure vanadium oxide systems.  相似文献   

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Aluminum–vanadium bimetallic oxide cluster anions (BMOCAs) have been prepared by laser ablation and reacted with ethane and n‐butane in a fast‐flow reactor. A time‐of‐flight mass spectrometer was used to detect the cluster distribution before and after the reactions. The observation of hydrogen‐containing products AlVO5H? and AlxV4?xO11?xH? (x=1–3) strongly suggests that AlVO5? and AlxV4?xO11?x? (x=1–3) can react with ethane and n‐butane by means of an oxidative dehydrogenation process at room temperature. Density functional theory studies have been carried out to investigate the structural, bonding, electronic, and reactive properties of these BMOCAs. Terminal‐oxygen‐centered radicals (Ot.) were found in all of the reactive clusters, and the Ot. atoms, which prefer to be bonded with Al rather than V atoms, are the active sites of these clusters. All the hydrogen‐abstraction reactions are favorable both thermodynamically and kinetically. To the best of our knowledge, this is the first example of hydrogen‐atom abstraction by BMOCAs and may shed light on understanding the mechanisms of C? H activation on the surface of alumina‐supported vanadia catalysts.  相似文献   

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The reactions of cerium–vanadium cluster cations CexVyOz+ with CH4 are investigated by time‐of‐flight mass spectrometry and density functional theory calculations. (CeO2)m(V2O5)n+ clusters (m=1,2, n=1–5; m=3, n=1–4) with dimensions up to nanosize can abstract one hydrogen atom from CH4. The theoretical study indicates that there are two types of active species in (CeO2)m(V2O5)n+, V[(Ot)2]. and [(Ob)2CeOt]. (Ot and Ob represent terminal and bridging oxygen atoms, respectively); the former is less reactive than the latter. The experimentally observed size‐dependent reactivities can be rationalized by considering the different active species and mechanisms. Interestingly, the reactivity of the (CeO2)m(V2O5)n+ clusters falls between those of (CeO2)2–4+ and (V2O5)1–5+ in terms of C?H bond activation, thus the nature of the active species and the cluster reactivity can be effectively tuned by doping.  相似文献   

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